Studies are being actively performed for enhancing the light emission efficiency of an organic light emitting device, such as organic electroluminescent device (organic EL device). In particular, various studies are being performed for enhancing the light emission efficiency by devising the material used in the light emitting layer. Among the studies, there are studies relating to an organic electroluminescent device containing a host material and a guest material (i.e., a light emitting dopant), in which the device emits light through migration of excitation energy formed in the host material to the guest material.
Patent Documents 1 and 2 describe an organic electroluminescent device using a host material, a light emitting dopant, and an assist dopant, as materials of a light emitting layer. In the organic electroluminescent device, the assist dopant complements the migration of carrier in the light emitting layer, and for example, a hole transferring material, such as a phenylamine derivative, is used therefor in the case where the transfer of electrons is complemented, and an electron transferring material is used therefor in the case where the transfer of holes is complemented. Patent Documents 1 and 2 describe that the use of the assist dopant increases the probability of the recombination of carrier, and enhances the light emission efficiency of the organic electroluminescent device.
Patent Document 3 describes an organic electroluminescent device using a first dopant that is formed of a material capable of converting triplet excitation energy to light emission and has a first energy gap, a second dopant that is formed of a material capable of converting triplet excitation energy to light emission and has a second energy gap that is larger than the first energy gap, and a host material that has a third energy gap that is larger than the second energy gap, as materials of a light emitting layer, and describes an organic metal complex having iridium as a center metal, as an example of the first dopant and the second dopant. Patent Document 3 describes that the use of the combination of the two kinds of dopants and the host material enhances the light emission efficiency of the organic electroluminescent device, lowers the driving voltage, and enhances the light emission lifetime.